1. FIELD OF THE INVENTION
The invention relates to a process and apparatus for the regeneration of fluidized catalytic cracking catalyst.
2 DESCRIPTION OF RELATED ART
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts. A good overview of the importance of the FCC process, and its continuous advancement, is reported in Fluid Catalytic Cracking Report, Amos A. Avidan, Michael Edwards and Hartley Owen, as reported in the Jan. 8, 1990 edition of the Oil & Gas Journal.
Modern catalytic cracking units use active zeolite catalyst to crack the heavy hydrocarbon feed to lighter, more valuable products. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, much less contact time is needed. The desired conversion of feed can now be achieved in much less time, and more selectively, in a dilute phase, riser reactor.
There has been considerable evolution in the design of FCC units, which evolution is reported to a limited extent in the Jan. 8, 1990 Oil & Gas Journal article. Many FCC designs have evolved.
The Kellogg Ultra Orthoflow converter, Model F, shown in FIG. 1 of this patent application, and also shown as FIG. 17 of the Jan. 8, 1990 Oil & Gas Journal article discussed above, is an example of a modern, efficient FCC unit. This design (and many other FCC designs not shown) converts a heavy feed into a spectrum of valuable cracked products in 2-10 seconds of residence time in a riser reactor. The units are very efficient for cracking heavy feeds but present some design challenges. One of the most challenging features is the ceramic plug valves used to control flow of solids within a vessel, where no slide valve can be located. These valves tend to stick, and large amounts of purge gas are needed to keep the interior parts of the plug valve free of catalyst.
Hollow stem plug valves, suitable for use in Orthoflow type units are, disclosed in U.S. Pat. No. 2,850,364 and U.S. Pat. No. 4,827,967, which are incorporated by reference.
Most refiners now use all riser cracking today, as compared to dense bed cracking which was the prevalent reactor design in the 40's through the 60's. Refiners know that short contact time riser cracking is beneficial and have tried to use conventional equipment to shorten residence time and minimize thermal cracking. Approaches include riser quenching and closed cyclones which reduce post-riser thermal cracking. Some refiners have dropped pressure to increase selectivity. Some have gone to short contact time cracking reactors, usually involving higher vapor velocities in the risers.
It is difficult to use conventional riser reactors and have short contact time by using high velocities because the higher velocities increase the erosive power of the FCC catalyst. High velocities are hard on the equipment, which is subjected to years of "sandblasting" and hard on the FCC catalyst, which attrits when it hits solid objects at high speed.
A limited amount of work has been done on extremely short contact time FCC reactors. Most of it involves downflow reactors and solving problems associated with intimately contacting hot regenerated catalyst with oil and then separating cracked products from spent catalyst within a second or less. Current state of the art risers have oil residence times as low as one second, while some proposed designs will operate with even less residence time in the riser.
U.S. Pat. No. 4,832,825 uses a riser of 5 to 40 meters and high velocity in the riser to achieve hydrocarbon residence times of 0.05 to 10 seconds. Material exiting the riser 1 is deflected down by a cap to effect some measure of spent catalyst and cracked product separation.
U.S. Pat. No. 4,919,898 Gartside et al, which is incorporated by reference, teaches a short residence time apparatus for cracking hydrocarbon with hot solids. An annular falling curtain of hot solids contacts opposing spray feed nozzles. Catalyst flow is controlled by changing the pressure (via steam injection) in a solids reservoir above a plug valve 14 having a spherical base portion with arcuate contours 15. There is fairly efficient formation of a large surface area annular sheet of catalyst, but the nozzle configuration shown will not ensure uniform catalyst and oil contact circumferentially around the contours 15. The nozzles are basically point devices, while the catalyst forms a plane, and perfect contact of a plane using a plurality of points is not possible. The catalyst and oil mixture passes down into a larger separation area, comprising a horizontal plate. Cracked products are withdrawn from above the plate so cracked gases follow a 180 degree path from inlet to outlet. The separator is reported to recover from 95 to 99% of the solids.
U.S. Pat. No. 4,433,984, which is incorporated by reference, teaches a short contact time cracking process, with rapid separation of solids from cracked gases leaving a cracking reactor.
U.S. Pat. No. 4,985,136, which is incorporated by reference, discloses a falling curtain FCC reactor, using very high zeolite content cracking catalyst (40 to 80% zeolite) to crack heavy feed within 0.5 seconds or less. Catalyst falls down, and oil is injected horizontally into a cyclone separator 66. A falling wall of catalyst is contacted by one or more sprays or jets of feed, so there will be some areas with high catalyst concentrations and low amounts of feed, and some areas with excessive amounts of hydrocarbon, especially if a nozzle malfunctions and develops a narrow jet or spray of feed rather than a more diffuse spray. The cyclone dipleg discharged catalyst into a large stripping section 10.
While many improvements have been made in developing an effective short contact time cracking process, none were entirely satisfactory. I was most concerned about two areas: initial contacting of catalyst (or other hot solids in the case of thermal processes) and oil, and the efficient separation of cracked products from spent catalyst (or coked solids).
The falling curtain concept is a good one but creates problems. Achieving an annular curtain or shower of catalyst provides maximum exposure of catalyst to oil, but such flows are hard to control accurately, especially so when a run length of years is contemplated. Formation of a wall or plane of falling catalyst, rather than an annulus, is simpler, but requires a thicker wall for a given amount of catalyst flux as compared to an annulus of catalyst in a vessel with the same diameter.
The rapid separation of catalyst, or solids, downstream of a short contact time reactor presents more challenges to the chemical engineer. The use of an enlarged separation section, with reactor products discharging down onto a horizontal plate in U.S. Pat. No. 4,919,898, forcing the cracked vapor to make a 180 degree turn to a vapor outlet, produces significant separation but is not optimum. Horizontal discharge into a cyclone inlet, as in U.S. Pat. No. 4,985,136, allows efficient separation but is hard on the cyclones and requires some work because of the pressure drop associated with the cyclone.
An additional problem with a resid cracking unit is higher temperature. The temperature of regenerated catalyst tends to increase as the FCC feed gets worse, and the higher temperatures associated with resid cracking (or short contact time cracking of VGO) weaken the metal used in most catalyst flow control valves.
I realized that most of the difficulties of the existing approaches to short contact time cracking, whether catalytic or thermal, could be overcome by using a plug valve to control flow of and distribute hot solids into an annular, falling curtain or sheet of catalyst, and simultaneously admit part or all of the feed through the radial opening under the plug valve seat. I realized that these conventional plug valves, which have proven highly reliable in years of service in conventional FCC units, could be used as part of a robust short contact time FCC unit or thermal cracking unit. Using the body of the valve to admit feed permitted a measure of preheating of the feed to be achieved and made a vice of these valves (their need for constant addition of purge gas) a virtuous way to add feed, which even cooled the valve. In a preferred design, I couple this new reactor design with an improved catalyst/vapor separator, providing for low impact separation which is also highly efficient.